formatting races with rustc, causing compile errors to have wrong locations

[programmerjake]

when using Expander::fmt, rustc often reads the generated file before rustfmt has finished running, causing the compile errors to be based on the unformatted code, making the locations not match the formatted code.

This is because expand_to_file uses Command::spawn instead of Command::status or similar and doesn't wait for rustfmt to finish before returning.

[drahnr]

Thank you for filing the issue! I encountered the issue very sporadically but didn't get around to debug it.

Would you'd like to create a PR? Otherwise I'll do it in a few hours.

Note that a failure to format should only emit a warning, and not break compilation

[drahnr]

@programmerjake could you give #10 a spin, it does work for me.

If I don't hear back from you by Monday, I'll cut a 2.1.0 release including PR #10.